Polytrimethylene ether glycols can be produced via the acid-catalyzed polycondensation of 1,3-propanediol, optionally in the presence of comonomer diols. Standard methods of producing polytrimethylene ether glycols using acid catalysis are described in U.S. Pat. Nos. 6,720,459, 3,326,985, and 2,520,733.
Polytrimethylene ether glycol polymers produced by the polycondensation of 1,3-propanediol can have quality problems, in particular high color, which makes them unsuitable for many end-uses, such as textile fiber applications. The color of the polymer has historically depended primarily on raw material quality and the polymerization process conditions, such as catalyst concentration and reaction temperature. Attempts to prepare higher molecular weight (i.e., greater than about 250 g/mole) polytrimethylene ether glycols that have low color generally have either been unsuccessful or required long reaction times under mild reaction conditions.
U.S. Patent Application No. 2005/0272911 discloses a method for producing a polyether polyol having reduced color by a dehydration condensation reaction using a catalyst comprised of both an acid and a base. The addition of a base can add to the cost of synthesis of the polymer, as well as to the cost of separation of the polymer from the reaction mixture.
Several recent attempts to reduce color have involved the use of post-reaction treatments. For example, U.S. Pat. No. 7,009,082 discloses a method for the removal of color bodies from polytrimethylene ether glycol by adsorption methods, and U.S. Patent Application 2004/0225163 discloses a method for color reduction by hydrogenation. These post-reaction methods are time consuming and require additional process equipment and chemicals, and are therefore expensive.
The present invention provides an improved process for preparing polytrimethylene ether glycol or its copolymers wherein the product polymers have an average molecular weight above at least about 250 g/mole and exhibit reduced color.